Multilevel structures, consisting of silicon layers separated by insulating dielectrics, are extensively used in integrated circuit structures such as charged coupled devices, EEPROM devices and CMOS devices. The insulating dielectric is typically silicon dioxide which is formed by thermal oxidation of the underlying silicon surface or by chemical vapor deposition thereon. Thermally grown silicon dioxide is generally preferred because of the simplicity of the processing involved and the purity of the oxide obtained.
Certain device structures require a thick layer of insulating oxide, i.e. at least 1,200 nanometers and, in certain instances, exceeding two micrometers in thickness. For example, layers of silicon dioxide utilized to separate functional entities in a multilayer structure, are frequently well in excess of one micrometer in thickness. Certain processes such as deposited silicon recrystallization utilize as a starting substrate a silicon wafer having a thick coating of silicon dioxide on its surface. Optical gratings are formed from a sapphire substrate having a thick layer of silicon dioxide on its surface, and the like.
There are a number of disadvantages inherent in forming a thick layer of silicon dioxide by conventional deposition processes, such as sputtering or electron beam evaporation. Thick layers of silicon dioxide formed by deposition are generally characterized by a significant stress factor. Such layers also often contain voids. The stress and voids problems are conventionally alleviated by high temperature annealing of the structure which may be disadvantageous depending on its ability to withstand high temperatures. There is also the possibility that impurities from the environment may be trapped in the growing film, particularly if it is deposited by sputtering techniques.
Growing of a thick layer of silicon dioxide by thermal oxidation also has certain disadvantages. Most prominent among these is that, as the layer of oxide gets thicker, it becomes increasingly difficult for the oxygen to reach the underlying silicon and, therefore, the growth process materially slows as the layer of oxide thickens. Under ideal conditions, it may require about seven hours in a steam ambient at 950.degree. C. as a single oxidation step to grow a layer of silicon dioxide 1,200 nanometers thick. In addition to the time and energy required, such an extended oxidation process at high temperature often causes diffusion of impurities in the substrate which can adversely affect both their profile distribution and the efficacy of their function. An extended oxidation can increase the density of oxidation-induced stacking faults which result from thermal oxidation of a silicon substrate. Experience has shown that such stacking faults become more pronounced as oxidation extends deeper into the substrate. Lastly, an extended oxidation of a silicon substrate which consumes the silicon can be disadvantageous in that, if not properly controlled, it may leave more or less than the desired thickness of a layer of silicon overlying a layer of different material, a buried device, and the like.
In accordance with this invention, a method has been found to form a thick layer of silicon dioxide by thermal oxidation of a silicon substrate which requires substantially less time than conventional thermal oxidation and which minimizes or eliminates the disadvantages associated therewith.